| John Day Strata | |
|---|---|
| Stratigraphic range: Eocene-Early Miocene | |
 Exposures of the Turtle Cove Formation along a hiking trail in the Sheep Rock Unit of the John Day Fossil Beds National Monument | |
| Type | Sedimentary (mostly from clasts of igneous origin), igneous |
| Underlies | Columbia River Basalt Group |
| Overlies | Clarno Formation |
| Lithology | |
| Primary | Sandstone, tuff |
| Other | Pyroclastic |
| Location | |
| Region | Central Oregon |
| Country |  United States |
| Type section | |
| Named for | John Day River |
The John Day Formation is a series of rock strata exposed in the Picture Gorge district of the John Day River basin and elsewhere in north-central Oregon in the United States. The Picture Gorge exposure lies east of the Blue Mountain uplift, which cuts southwest–northeast through the Horse Heaven mining district northeast of Madras. Aside from the Picture Gorge district, which defines the type, the formation is visible on the surface in two other areas: another exposure is in the Warm Springs district west of the uplift, between it and the Cascade Range, and the third is along the south side of the Ochoco Mountains. All three exposures, consisting mainly of tuffaceous sediments and pyroclastic rock rich in silica, lie unconformably between the older rocks of the Clarno Formation below and Columbia River basalts above.[1]